1. Field of the Invention
The present invention relates generally to optical reading apparatus, and more particularly is directed to a device for supporting an objective lens in apparatus for optically reading an audio or video signal or the like recorded in a track on a record medium.
2. Description of the Prior Art
In a prior art optical reading apparatus shown on FIG. 1, an optical record medium surface 1 has an information signal, such as, an FM-modulated video signal or a PCM audio signal or the like recorded thereon as a pit array in a spiral track or in a series of concentric circular tracks. A laser light souce 2 emits a laser beam LB, which is reflected by a mirror 3a of a tracking mirror assembly 3 onto the record medium 1. The beam passes from mirror 3a through an objective lens 5, which is part of a focusing assembly 4. If the laser light source is a semiconductor device, the laser beam reflected from the record medium 1 may be returned along the illustrated path directly to the laser light source 2. If the laser light source 2 is of another type, such as, a gas laser, a beam splitter (not shown) is provided to direct the reflected laser beam to a photoelectric transducer (not shown). In either case, the laser beam reflected from the surface 1 of the record medium is modulated with the information recorded thereon.
The mirror 3a of tracking mirror assembly 3 is seen to be mounted, at the center of its back surface, by a rubber or other flexible member 6 to a fixed support 7. Magnets 8 are attached to the back surface of mirror 3a at opposed locations, and fixed coils 9 are disposed in juxtaposed relation to magnets 8. A tracking error signal is supplied to the fixed coils 9 which electromagnetically cooperate with magnets 8 to angularly displace mirror 3a relative to its fixed support 7 for causing the laser beam LB to accurately scan the track in which the information signal is recorded.
The focusing assembly 4 of the apparatus of FIG. 1 includes an electromagnetic device known as a linear motor. As shown, the objective lens 5 is attached to a cylindrical bobbin 10 on which is wound a coil 11, with bobbin 10, lens 5, and coil 11 forming a moving member 12. A cylindrical guide member 12a depends from bobbin 10 about the periphery of lens 5 for mechanically locating moving member 12 relative to a magnetic yoke 14 which includes an annular bottom portion and inner and outer cylindrical portions inside and outside bobbin 10. A bearing 16 is carried by the inner cylindrical portion of yoke 14 for guiding cylindrical guide member 12a. Included in the outer cylindrical portion of yoke 14 is an annular magnet 13. The magnet 13 and yoke 14 together form a fixed member 15, and moving member 12 and fixed member 15 together form the linear motor of focusing assembly 4. A focusing error signal is suitably supplied to coil 11, for electromagnetically moving moving member 12 vertically with respect to fixed member 15 in the direction to restore the correct focusing of laser beam LB at surface 1.
The above prior art optical reading apparatus has at least one substantial drawback. Angular displacement of tracking mirror 3a changes the angle between laser beam LB and the optical axis of lens 5. If it is desired to provide an objective lens 5 which is free from aberration as a result of this change of angle, the number of lenses that are required to form objective lens 5 must be increased to correspondingly increase its cost.
A further drawback is that the rubber connecting member 6 of tracking assembly 3 deteriorates with time and, in particular, begins to have hysteresis characteristics, which impair the accuracy of tracking. In addition, providing a tracking assembly 3 separate from the focusing assembly 4 increases the size and difficulty of installation of the optical reading apparatus.
Another example of a prior art optical reading apparatus is disclosed in U.S. Pat. No. 4,135,083. In such optical reading apparatus there is no tracking mirror. Rather, focusing and tracking corrections are effected by moving the objective lens in both the direction of the optical axis (for focusing) and a direction perpendicular to the optical axis (for tracking). In this apparatus, however, the mass of the movable member which includes the objective lens is relatively large. Thus, in order for the movable member to have the proper kinetic characteristics for responding to tracking and focusing error signals, it is necessary for either the driving current or the number of coil turns in its associated electromagnetic driving means to be relatively large.